The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventor, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Contact image sensor (CIS) devices generally include an array of sensors and an illuminator. The array of sensors typically cannot differentiate between colors. Even so, CIS devices can be used to scan media, such as a sheet of paper having text, and determine that colors are present on the media. To do so, the illuminator illuminates the media with primary colors one at a time, such as first with red, then with green, and then with blue. During each color illumination, the CIS sensors detect reflected photons but not their color. Because the device's controller knows which color is being illuminated for each set of detections, the controller can determine which colors are on the media.
CIS devices, however, move relative to the media and often can only do so once. When a sheet of paper is fed through a copier, for example, the sheet of paper is scanned by a CIS device with one, single pass. Because of this, the CIS sensors cannot detect all three primary colors at same locations on the paper. Instead, the colors are illuminated and sensed in succession for each scan line. A scan line is generally 1/600th of an inch and there are typically three colors used. Thus, for each third of 1/600th of an inch of the media, a different color is being sensed. This can result in inaccurate scans.
Consider a case where a white sheet of paper having black text is being scanned. If the illuminator is illuminating the paper with red at the transition between the white and the black, and thus on the last bit of white paper before the black text starts, the CIS sensors will detect reflected photons, which the CIS controller will interpret as red. The illuminator then proceeds to illuminate with green, which is absorbed by the black text, in response to which the CIS sensors do not detect reflected photons. The same is true for the blue that follows next, and then the red following thereafter, and so forth until the black text ends. Thus, in this example, a red-colored fringe is sensed at the white-to-black transition. Conventional CIS devices suffer from this and various other inaccuracies, which can result in a copier printing a page with a color fringe that is not present on the original. Not only is this inaccurate, it also can substantially increase costs and copying times.